Projection displays conventionally comprise a two-dimensional array of light emitters and a projection lens. The lens forms an image of the array at some plane in space, and if this imaging plane is far from the projection lens, so that the light rays are more or less parallel, then the effect of the projection lens is essentially to collimate light from any pixel on the two-dimensional array.
Projection displays are most commonly configured so that the image of the array falls on a large translucent screen, and a viewer looking at the screen will see a greatly magnified image of the picture on the two-dimensional array. However, it is becoming increasingly common for small projection displays to be mounted on the head of a viewer so that the projection display is directed towards the viewer's eye, and light collimated by the projection lens from a single pixel on the two-dimensional array of light emitters is subsequently focused by the viewer's cornea onto the retina so that the viewer sees an apparently distant image often known as a virtual image.
It is also possible to place a large-diameter projection display comprising a two-dimensional array of directional light emitters behind a liquid-crystal display or some other spatial light modulator in order to synthesize a three-dimensional image. See, for example, Travis, A. R. L., “Autostereoscopic 3-D display”, Applied Optics, Vol. 29, no. 29, pp. 4341–3. One pixel at a time of the two-dimensional array of light emitters is illuminated, and an appropriate view of a three-dimensional object is simultaneously displayed on the liquid-crystal display in such a way that the view of the three-dimensional object is only visible if observed from the direction in which the rays of light collimated by the projection lens from the pixel are traveling. A sequence of views is repeated at a rate faster than that at which the eye can detect flicker, thereby time-multiplexing a three-dimensional image.
This display is three-dimensional but not holographic. It is possible in principle to create a holographic three-dimensional image by placing a two-dimensional array of point-source light emitters in the focal plane of the projection lens, illuminating each point source in turn, and displaying appropriate holograms on a liquid-crystal display placed on top of the projection lens so that each hologram is made visible to a different point of view in turn.
Head-mounted displays are bulky and users would prefer that they were flat. A head-mounted display can be made flatter using a slab waveguide incorporating a weak hologram, as shown by Amitai, Reinhorn and Friesem, “Visor-display design based on planar holographic optics,” Applied Optics, Vol. 34, No. 8, pp. 1352 to 1356, 10 Mar. 1995. Light from a cathode-ray tube and hologram is coupled into the waveguide, and this light will be diffracted out of the waveguide (i.e. normal to the slab) by the hologram in directions which are determined by the pixel within the cathode-ray tube from which the light was emitted.
Three-dimensional images synthesized by time-multiplexing the illumination of a liquid-crystal display require the liquid-crystal display to have a fast-switching array of thin-film transistors and these are expensive. Trayner and Orr in U.S. Pat. No. 5,600,454 describe a device which avoids this by placing a hologram behind a conventional liquid-crystal display that directs the illumination of alternate rows to a left-eye or right-eye view. But both this and the switched-illumination concept are bulky, and do not exhibit the flatness needed for head-mounted displays.
Instead, a flat-panel three-dimensional display can be made by combining a projection display with a screen from which light shone parallel to the surface of the screen is ejected at one of a set of selectable lines along the screen, as described in the inventor's earlier application PCT/GB 97/02710 (WO 98/15128). One line at a time on the screen is selected, and simultaneously the projection display projects a line of pixels parallel to the screen so that they are ejected at the selected line. The same line of pixels on the projection display is altered repeatedly as each of the series of lines on the screen is selected in turn in such a way as to time-multiplex a complete image on the screen. Only one line of the projection display is used, so the array of light emitters need be only one line high, and if the emitted light is collimated in the plane of the screen then the projection lens need be only one or two millimeters high so that the combined projector and screen are flat.
If it is light from a three-dimensional display, albeit one whose array of light emitters is only one pixel high, that is directed parallel to the surface of the screen of selectable lines, then the image formed on the screen is three-dimensional. The three-dimensional display might comprise an array of light emitters behind a projection lens with a liquid-crystal display in front of the projection lens, as described above, but in order to put up several views within one line period of the display the switching rate of the liquid crystal would need to equal the number of views times the line rate of the display, and few liquid-crystal mixtures switch this fast.
Many other kinds of autostereoscopic and holographic three-dimensional display concepts exist and any could be used in a flat-panel system. Particularly interesting is an old concept comprising a group of small video projectors in the focal plane of a field lens. Each projector is positioned to form a view in the plane of the field lens just as if the lens were a translucent screen, but unlike a translucent screen the field lens collimates the light so that the picture is visible from only a single direction. The other projectors form views which are made visible by the field lens to other directions so that the viewer sees an autostereoscopic three-dimensional image. However, viewers prefer three-dimensional images to be autostereoscopic both in azimuth and in elevation, and little consideration has been given with this concept to making views vary with elevation.